1. Field of the Invention
This invention relates to a semiconductor memory device, and more particularly to a shorted junction type programmable read only memory (PROM) semiconductor device.
2. Description of the Prior Art
In a shorted junction type PROM semiconductor device, a destroy or breakdown phenomenon, at a junction, is uniquely utilized to provide a binary state corresponding to an information. Such PROM semiconductor device is disclosed for example in "Digest of Technical Papers" of The 11th Conf. (1979 International) on Solid State Devices, Tokyo, Pages 75 and 76, Aug. 27-29, 1979.
The PROM semiconductor device disclosed therein consists of a monocrystalline silicon substrate and an NPN junction structure, for example, formed in the longitudinal direction. This PROM semiconductor device is in a relatively high impedance state with a normal bias (for example, 5 V), that is, in a state not to enable writing of an information. To write an information in the PROM semiconductor device, a high voltage (for example 20 V) is applied across the junction to create a breakdown phenomenon so as to pass a large current through the junction to heat the same, thereby diffusing an electrode metal or an impurity in the semiconductor to destroy or short the junction to cause a low impedance state.
In the PROM semiconductor device of this construction, however, the write voltage is determined by the breakdown voltages of the individual junctions of the device and the breakdown voltages vary depending upon the impurity concentration of the semiconductor device or the configurations of the junctions. Accordingly, when the write voltage is set to a critical value, it is necessary to vary the write voltage in accordance with the non-uniformity of the breakdown voltage of the PROM device. For this reason, the write voltage is generally set to a value substantially higher than the breakdown voltage which is determined by taking into consideration the non-uniformity of the characteristics.
Of course, it is possible to decrease the breakdown voltage by reducing the breakdown strength of the junction for the purpose of lowering the write voltage with this measure. However, the possibility of erroneous writing of the information increases due to non-uniformity of the characteristics of the device of the conventional construction. When the write voltage is made higher, as above described, the design of the peripheral circuits becomes difficult which increases power consumption at the time of writing, thus making it impossible to save electric power.
In the prior construction described above, it is a common practice to form a PN junction in the semiconductor substrate. With this construction, however, the electric power supplied, at the time of writing for breaking down the junction, is used to also heat the semiconductor substrate itself, so that, in order to generate heat at the junction, in a quantity sufficient to destroy or short the same, it is necessary to use an excess amount of power, which is, of course, undesirable.